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United States Patent |
5,675,335
|
Keller
|
October 7, 1997
|
Method of improving the distortion behavior of analog-to-digital
converters
Abstract
In a method of improving a distortion ratio of an analog-to-digital
converter, an analog signal to be digitized is additively superposed with
a band-limited noise signal. The thusly produced sum signal is supplied to
the analog-to-digital converter for sampling and quantization.
Inventors:
|
Keller; Helmut (Pfullingen, DE)
|
Assignee:
|
Wandel & Goltermann Elektronische Messtechnik GmbH (Horb, DE)
|
Appl. No.:
|
528948 |
Filed:
|
September 15, 1995 |
Foreign Application Priority Data
| Apr 13, 1995[DE] | 195 14 007.9 |
Current U.S. Class: |
341/131; 341/144; 341/155 |
Intern'l Class: |
H03M 001/06 |
Field of Search: |
341/131,144,155
|
References Cited
U.S. Patent Documents
3999129 | Dec., 1976 | Kasson | 325/42.
|
4187466 | Feb., 1980 | Kasson et al. | 325/42.
|
4831464 | May., 1989 | Chijiwa | 360/32.
|
4965668 | Oct., 1990 | Abt et al. | 358/160.
|
5148163 | Sep., 1992 | Frindle | 341/131.
|
5525984 | Jun., 1996 | Bunker | 341/131.
|
5530442 | Jun., 1996 | Norsworthy et al. | 341/131.
|
Primary Examiner: Gaffin; Jeffrey A.
Assistant Examiner: Kost; Jason L. W.
Attorney, Agent or Firm: Striker; Michael J.
Claims
What is claimed as new and desired to be protected by Letters Patent is set
forth in the appended claims:
1. A method of improving the distortion behavior of an analog-to-digital
converter, comprising additively superposing an analog information signal
with a noise signal to produce a sum signal; subsequently supplying the
sum signal to an analog-to-digital converter so as to sample and digitize
the sum signal; and band-limiting the noise signal to a frequency range
located outside an information frequency range before said additively
superposing the analog information signal with the noise signal, said
limiting including selecting a RMS value of the noise signal substantially
greater than a RMS value of a quantization noise of the analog-to-digital
converter.
2. A method as defined in claim 1, wherein said limiting includes selecting
a RMS value of the noise signal greater than that of the information
signal.
3. A method as defined in claim 1, wherein said limiting includes selecting
the band-limited noise signal in a frequency range which is located
approximately in a region of half the sampling frequency of the
analog-to-digital converter.
4. A method as defined in claim 1; and further comprising the step of
changing the level of the noise signal by an adjustable amplifier or
attentuator to fit different applications, with low noise levels resulting
in high allowable information signals levels and high noise levels
resulting in lower harmonic distortion.
Description
BACKGROUND OF THE INVENTION
The present invention relates to a method of improving the distortion
behavior of analog-to-digital converters.
More particularly, it relates to a method of improving the distortion
behavior in accordance with which an analog information signal is
additively superposed with a noise signal before supplying it to an
analog-to-digital converter in which the sum signal including the
information signal and the noise signal is sampled and digitized.
In this document, the term "distortion" is used mainly for harmonic
distortion. Analog-digital converters in general generate a large number
of harmonics, especially as they generate harmonics with very high order.
These harmonics will be aliased into the frequency range below half of the
sampling frequency. The harmonic relationship to the fundamental frequency
is therefore often not obvious. These aliased harmonics of high order are
often called spurious signals. Thus, the term "distortion" is used for
this kind of distortion to.
The distortion behavior of analog-to-digital converters is clearly
different from that which is conventional for analog components. The
characteristic of an analog-to-digital converter can be described well as
an irregular stepped curve. The level of the distortion products in first
approximation independent from the control of the analog-to-digital
converter, so that the distortion factors increase with reducing the level
of the information signal. The level of the individual distortion products
is located typically in the region of several decibels under the wide-band
quantization noise level. With a real 12-bit converter the individual
distortion products have a level of less than about 70 dBFS, wherein
"dBFS" is a measure for the level, referred to the full scale level of the
analog-to-digital converter. With analog components such as amplifiers or
mixers, the distortion factors to the contrary decrease with reducing the
level of the information signal, so that with sufficiently small levels
very small distortion factors can be obtained.
In order to avoid the disadvantages of the quantization effect of
analog-to-digital converters it is a current practice to utilize so-called
"Dither" signals. These signals are additively superposed on the
information signal and serve for modulating the converter, also during
small information signal levels, of slightly more than 1 LSB (1 LSB is the
height of a quantization stage). Most often white noise is utilized as the
Dither signal. With this method also information signals with amplitudes
under 1 LSB can be evaluated. The level of the Dither signal is maintained
as low as possible so that the signal to noise ratio and the control range
of the converter are not substantially worsened. A substantial improvement
of the distortion behavior in the case of large information signals cannot
be obtained through these current methods.
If it is necessary to improve the distortion behavior for larger
information signals, the level of the Dither signal must be also selected
high. This however is accompanied by disadvantages. A method of
utilization of this approach is known. White noise produced by a digital
random generator is utilized as a Dither signal. A very high quality DA
converter (digital-to-analog converter) converts the digital noise signal
into an analog signal. The analog noise signal is additively superimposed
with the information signal. The level of the noise signal is so high that
the control range for the information signal is substantially reduced (the
sum of the Dither signal and the information signal must be located under
the control limit of the AD converter). The wide-band, white Dither signal
drastically worsens the signal to noise ratio. For eliminating this
disadvantage, the digital noise signal is subtracted from the digitized
sum signal by a digital subtractor. In order to perform this noise
compensation well, a very high quality DA converter must be utilized.
Moreover, the analog summing amplifier must operate in a very precise way.
This method is very expensive and requires very high quality components.
SUMMARY OF THE INVENTION
Accordingly, it is an object of the present invention to provide a method
of improving the distortion behavior of analog-to-digital converters,
which avoids the disadvantages of the prior art.
In keeping with these objects and with others which will become apparent
hereinafter, one feature of the present invention resides briefly stated,
in a method of improving the distortion behavior of analog-to-digital
converters, in accordance with which an analog information signal is
superposed with a band-limited noise signal, and the frequency range of
the noise signal is selected so that it is located outside of the
information frequency range.
In this way the noise signal does not worsen the noise behavior of the AD
converter in the information frequency range. The noise components outside
of the information frequency range in many applications such as spectral
analysis are of no importance. If a wide-band evaluation is necessary,
these spectral components can be easily suppressed by a simple digital
filter, and only several percentages of useable band width are lost. The
level of the noise signal can be selected in this way so high as it is
allowed by the control limit of the AD converter without worsening the
noise behavior of the AD converter in the information frequency region.
However, care should be taken that the sum of the information signal and
the noise signal is located below the control limit of the AD converter.
Short term overdrivings which occur relatively seldom and are not very
strong do not worsen the distortion behavior and the noise behavior.
Particularly when the information signal is smaller than the noise signal,
the distortion behavior of the AD converter is improved dramatically. When
the level of the noise signal is adjustable the distortion behavior and
the control range for the information signal can be optimized for
different applications.
It has been shown by experiments that the proposed new method is at least
of the same quality as the above described and substantially more
expensive method with respect to the distortion behavior. The noise
behavior of the proposed method is in the practice better than the noise
behavior of the known method since no noise compensation is needed.
For obtaining a great intermodulation free dynamic range in the IF stage of
spectrum analyzers, it is advantageous to superpose additively on the
information signal a large noise signal in the frequency range of half the
sampling frequency. The RMS value of the noise signal can be greater than
that of the information signal. It has been shown by experiments that with
a resolution band width of 1 kHz an intermodulation free dynamic range of
over 80 dB can be realized with an intermediate frequency of 19.2 MHz with
fully digital intermediate frequency evaluation. The intermodulation free
dynamic region with 1 Hz resolution band width can be located over 100 dB.
A further positive effect of the method in accordance with the present
invention is that the level linearity of the analog-to-digital converter
becomes approximately ideal. This means that the accuracy of selective
level measurement does not depend on the signal level.
The novel features which are considered as characteristic for the invention
are set forth in particular in the appended claims. The invention itself,
however, both as to its construction and its method of operation, together
with additional objects and advantages thereof, will be best understood
from the following description of specific embodiments when read in
connection with the accompanying drawing.
BRIEF DESCRIPTION OF THE DRAWING
The FIGURE of the drawing is a view showing a block diagram illustrating a
method of improving the distortion behavior of an analog-to-digital
converter in accordance with the present invention.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
In accordance with a method of improving the distortion behavior of an
analog-to-digital converter, a sum signal composed of an analog
information signal AS and a band-limited noise signal RS is supplied to an
input of an analog-to-digital converter ADC. The noise signal is produced
by a noise signal generator RSG and a subsequently arranged band pass
filter BP. The level of the noise signal can be adjustable by an
adjustable amplifier or attentuator V. A digitized signal DS is produced
at the output of the analog-to-digital converter ADC.
The noise signal RS is limited to a frequency range which is located
outside of an information frequency range. The level of the noise signal
RS is preferably greater than the level of the information signal AS.
However, it must be taken into consideration that the level of the sum
signal composed of the information signal AS and the noise signal RS is
located below the control limit of the analog-to-digital converter ADC.
The analog-to-digital converter ADC can be a 12 bit converter with a
sampling frequency of approximately 25.6 MHz. With such a selected
sampling frequency, the noise signal can have a center frequency of 12.8
MHz. The frequency band of the noise signal can be limited so that below
10 MHz and beyond 15.6 MHz the level of the noise signal is located at
least 12 dB under the converter noise.
It will be understood that each of the elements described above, or two or
more together, may also find a useful application in other types of
methods differing from the types described above.
While the invention has been illustrated and described as embodied in a
method of improving the distortion behavior of analog-to-digital
converters, it is not intended to be limited to the details shown, since
various modifications and structural changes may be made without departing
in any way from the spirit of the present invention.
Without further analysis, the foregoing will so fully reveal the gist of
the present invention that others can, by applying current knowledge,
readily adapt it for various applications without omitting features that,
from the standpoint of prior art, fairly constitute essential
characteristics of the generic or specific aspects of this invention.
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